Paul starts by drawing a line in the sand and daring his readers to cross it.

          Paul, an apostle--sent not from men nor by man, but by Jesus Christ and God the Father (Gal 1:1).

          This is the ultimate in "oneupmanship."  He doesn't care what authorities he is facing, he doesn't care if the teachers he opposes came from Jerusalem itself.  He has a 'higher authority,"  actually the highest of all.  He pulls rank on all his critics:  "Don't mess with me, pilgrim.  I come from the Presence of the Most High God."  He is not open to discussion, to negotiation, to compromise, because Eternal Truth is at stake.  Even the supernatural witness of an angel will not sway his doctrine (Gal 1:8).  He re-emphasizes this later in the first chapter:

          I want you to know, brothers, that the gospel I preached is not something that man made up. I did not receive it from any man, nor was I taught it; rather, I received it by revelation from Jesus Christ (Gal 1:11-12).
 

Paul is not bragging here, but defending his calling, which the Judaizers had undercut.  They could have claimed that Paul was not a legitimate apostle -- he had never been a disciple, never known Jesus prior to the Crucifixion, was not appointed by the Jerusalem leadership, etc.  To them, he must have seemed a lone wolf, and his doctrine was suspect.

Paul's dramatic conversion is the kind of phenomenon which Rufus Moseley called the experience of a "firsthander."  These are people who receive a direct revelation of Jesus Christ without the mediation of Scripture or Church or human witness.  This is, of course, more the exception than the rule in Christian history, and indeed beset with all sorts of fanaticisms and excesses. Yet it is a real and valid occurrence, and not limited to the first century.  Indeed, such experiences have been widely reported in recent years, particularly in places remote from regular forms of evangelism:  deathbeds, jails, closed countries, remote tribes.  Sadhu Sundar Singh of India was one early 20th Century example of this.  God is not limited to our frail attempts to communicate His truth, sometimes He steps in to take a more direct role.  It is at these times that His sovereignty is manifested, as Paul himself says:

          But when God, who set me apart from birth and called me by his grace, was pleased to reveal his Son in me... (Gal 1:15-16).